Aluminum alloy generally falls in strength the higher the temperature. Therefore, to maintain the high temperature strength in an aluminum alloy which is used for an automobile piston or under other high temperature conditions, in the past the practice has been to increase the amounts of addition of Si, Cu, Ni, Mg, Fe, Mn, etc. and increase the precipitates of second phase particles etc.
Among these elements which are added for improving the high temperature strength, Mn is added for improving the properties of the Fe-based compound. An Fe-based compound is effective for improvement of the high temperature strength, but tends to coarsen in a needle shape. If coarsening, the mechanical properties fall. For this reason, the practice has been to add Mn to convert the Fe-based compound to an α-form (for example, see Japanese Patent No. 4075523 and Japanese Patent No. 4026563).
Further, on the other hand, when increasing the amount of addition, the precipitates coarsen and become starting points of easy breakage, so the room temperature strength falls. Therefore, for example, as seen in Japanese Patent Publication No. 2007-216239 A1, to reduce the drop in the room temperature strength, when casting the aluminum alloy, the practice has been to treat the aluminum alloy melt ultrasonically at a temperature of the liquidus line or more so as to suppress the formation of coarse intermetallic compounds, that is, to make the structure finer.